In related art, there is known a cooling water passage structure for an engine as below. This structure includes cylinders arranged in a V-shape and a cylinder block having a water jacket surrounding the cylinders. A thermostat chamber is concavely provided in a valley portion in the upper surface of the cylinder block. An impeller chamber of a water pump is concavely provided at an end face of the cylinder block. A suction water passage connecting a bottom of the thermostat chamber with the impeller chamber, a cooling water passage connecting a water pump outlet with the water jacket, and a bypass hole communicating from the thermostat chamber to the outside are provided inside the wall body of the cylinder block. A warm water tube communicating from the water jacket to a radiator and a cooling water tube communicating from the radiator to the thermostat chamber are provided outside the cylinder head. (See e.g. Japanese Utility Model Laid-open No. Hei 2-139325.)